Fuel cells are attracting great attention due to their high energy conversion efficiency and low pollution emission, relative to conventional combustion engines and other sources of power. Proton-exchange membrane fuel cells (PEM-ECs) are promising power generators for portable, stationary, and automotive applications. A PEM-FC includes a membrane electrode assembly, composed of a cation (proton) exchange membrane with catalyst powder electrodes attached to the opposing membranes surfaces. The electrodes include catalyst particles and an electrode binder. A proton exchange membrane is also sometimes referred to as a polymer electrolyte membrane.
Oxygen permeation through the cathode layer of the membrane electrode assembly is important for fuel cell operation. Oxygen molecules need to permeate through the electrolyte layer to the surface of the catalyst, where the oxygen molecules are activated by the cathode catalyst to react with protons and electrons to form water through the oxygen reduction reaction. The rate of oxygen permeation through the cathode is apparently the control step of the oxygen reduction reaction, so that increased oxygen permeation increases the activation rate of the oxygen reduction reaction, directly increasing fuel cell performance.
In a fuel cell, hydrogen is split into protons and electrons at the anode (negative electrode), and the protons are transported through the electrolyte (such as a proton-exchange membrane) to the cathode (positive electrode). An electrode binder that is a good proton conductor is useful for transporting protons through the electrode to the catalyst. Hence, an electrode binder that has one or more of good oxygen permeability, good dimensional stability, high proton conductivity, high electron conductivity, and oxygen activation is desirable.
For hydrogen/air and direct methanol fuel cells, a typical electrode binder is a perfluorosulfonic acid polymer such as Nafion® (DuPont). Nafion® is attractive as a binder because it possesses a number of desirable properties, including chemical stability, mechanical stability, high proton conductivity, and high gas permeability to oxygen (air) and hydrogen. Unfortunately, Nafion® is an expensive material due to its complicated manufacturing procedure. Also, there is a serious environmental issue of HF release upon its decomposition under fuel cell operating conditions, which would be avoided using a non-fluorinated polymer. These problems, and others, have impeded the commercialization of PEM fuel cells. Hence, much effort has been devoted to the development of a cheap non-fluorinated membrane for the application in PEM fuel cells. See J. Roziere and D. J. Jones, “Non-fluorinated Polymers Materials for Proton Exchange Membrane Fuel Cells”, Annu. Rev. Mater. Res., 33, 503-503 (2003); M. A. Hickner et al., “Alternative Polymer Systems for Proton Exchange Membranes”, Chem. Rev, 104, 4587-4612 (2004). Although there has been significant research into new proton-exchange membrane materials for PEM fuel cells, there has been little research into finding alternatives to Nafion® for the electrode binder.